A urea synthesis solution obtained by reacting ammonia and carbon dioxide under high temperature and high pressure contains urea generated, water, unreacted ammonia and carbon dioxide. The urea synthesis solution is subjected to stripping or distillation sequentially under reduced pressure to separate unreacted substances, and unreacted ammonia and carbon dioxide are respectively contained in an amount of about 1% by mass or less before entering the concentration step, while the water content is 20 to 40% by mass. In the concentration step, water in the urea synthesis solution is evaporated. However, a small amount of unreacted ammonia and carbon dioxide are also contained in the drainage (evaporated water) of this concentration step. In addition, there are many cases where also urea is mixed even though it is slight. Therefore, a treatment to recover each component from the drainage containing urea, unreacted ammonia and carbon dioxide is required.
For example, Patent Document 1 discloses a treatment method consisting of a step in which an aqueous solution containing urea, ammonia and carbon dioxide is stripped in a first stripper to separate ammonia and carbon dioxide, thereby obtaining a urea aqueous solution containing substantially no ammonia, a step in which urea in this urea aqueous solution is hydrolyzed and a step in which the aqueous solution after hydrolysis is stripped in a second stripper to separate ammonia and carbon dioxide.
The treatment method of Patent Document 1 specifically includes the steps shown in FIG. 3. First, drainage consisting of urea (U), ammonia (N), carbon dioxide (C) and water (H) is introduced into a first stripper (PCS1) from a process condensate tank (PCT). By stripping with this first stripper (PCS1), ammonia (N) and carbon dioxide (C) are separated and discharged as a gas. The drainage consisting of urea (U) and water (H) after stripping (however, also trace amount of ammonia (N) actually remains) is introduced into a urea hydrolyzer (UHY). In this urea hydrolyzer (UHY), urea (U) is hydrolyzed to give ammonia (N) and carbon dioxide (C). Then, the drainage consisting of ammonia (N), carbon dioxide (C) and water (H) after hydrolysis is introduced into a second stripper (PCS2). Here, ammonia (N) and carbon dioxide (C) are separated by performing stripping again. As illustrated in the drawings of Patent Document 1, the first stripper (PCS1) and the second stripper (PCS2) are respectively arranged as an upper region and a lower side region of one tower, and ammonia (N) and carbon dioxide (C) separated in the second stripper (PCS2) are introduced into the upper first stripper (PCS1).
In addition, the urea hydrolyzer (UHY) requires to be given the heat necessary for the hydrolysis reaction. Thus, in Patent Document 1, heating steam (STM) is supplied to the urea hydrolyzer (UHY). Further, in Patent Document 1, the first stripper (PCS1) and the second stripper (PCS2) are arranged as an upper region and a lower region of one tower, and indirect heating using a reboiler and low pressure steam and direct heating by low pressure steam and steam from the second stripper are explained for the first stripper (PCS1) and direct heating by low pressure steam and indirect heating using a reboiler are explained for the second stripper (PCS2).
Patent Document 1 describes an effect that the hydrolysis efficiency of urea in the urea hydrolyzer (UHY) is improved since ammonia (N) and carbon dioxide (C) are separated in the first stripper (PCS1) before hydrolyzing urea (U) in the urea hydrolyzer (UHY). It is also described as one object of the invention to reduce the amount of heating steam necessary for processing.
In this treatment method of Patent Document 1, the hydrolysis efficiency in the urea hydrolyzer (UHY) is improved, and as a result, the amount of heating steam (STM) supplied to the urea hydrolyzer (UHY) is reduced. In particular, as compared with heating steam (STM) supplied to each stripper, heating steam (STM) supplied to the urea hydrolyzer (UHY) has higher temperature and higher pressure, so it is very important to reduce the use amount thereof. Therefore, if it is possible to further improve the treatment method of Patent Document 1 and to find a method of further reducing the amount of heating steam (STM) with high temperature and high pressure, it would be a very useful invention in the industry.
On the other hand, a method to recover ammonia in a gas discharged from the granulation step is known, for reducing emission of ammonia generated from a urea granulation step using a prill urea granulation equipment or a urea granulation equipment using a rotary drum, a fluidized bed or a fluidized-spouted bed into atmospheric air. For example, Patent Document 2 describes a method in which a gas containing urea dust and ammonia generated from a urea granulation equipment is introduced into a first scrubbing tower (corresponding to urea dust scrubbing equipment) in which a urea aqueous solution circulates, where the urea dust is chiefly recovered, then, the urea dust-recovered gas is introduced into a second scrubbing tower (corresponding to acid scrubbing equipment to be described later) in which an acid-containing water circulates, where ammonia is recovered.
In the urea dust scrubbing equipment, water is evaporated together with discharged air, so it is necessary to supply water from the outside. As water to be supplied to it, water generated as a by-product of urea synthesis is usually used. However, ammonia is contained in the water obtained by concentrating the urea synthesis solution, and if this water is supplied to the urea dust scrubbing equipment as it is, the amount of ammonia emitted into atmospheric air will undesirably increase. Therefore, it is usual to use water after treating with a stripper and a urea hydrolyzer, that is, water after thoroughly removing ammonia. The reason why the urea hydrolyzer is used here is that it is difficult to sufficiently remove ammonia in the presence of urea in water.
That is, conventionally, it is usual to use water after treatment with at least a urea hydrolyzer as water to be supplied to the urea dust scrubbing equipment, and the idea of using water before treating with a urea hydrolyzer is not common.